1. Field of the Invention
The present invention relates to a thermal resistance calculation method and device for calculating the thermal resistance of a semiconductor package in which a semiconductor chip is incorporated within a case.
2. Description of the Related Art
Among the problems such as failures in operation that typically occur in electronic components, problems originating from heat generally occur due to increase in localized temperatures in electronic components. With the increasing density of electronic components that has accompanied the decreasing size of electronic equipment in recent years, temperature can increase significantly within a short range. In other words, these problems are caused by increase in calorific density. Conventionally, rough estimates of temperature are made for each electrical component in the design stage and components are then fabricated with due consideration given to temperature variation. With the increase in heat density, however, the actual heat conditions may exceed the temperatures estimated in the design stage, and problems including malfunctioning and failure may therefore occur.
For electrical parts mounted in electrical equipment, therefore, temperature estimates must be made stringently for each individual electrical component, and the design of cooling devices (including the choice and placement of heat radiation fins, fans, etc.) must be based on these temperature estimates. Thermal resistance must be accurately determined to predict temperatures in semiconductor packages, which are electrical parts that generate heat. One method of the prior art for precisely determining thermal resistance in a semiconductor package is disclosed in "Validation Study of Compact Thermal Resistance Models of IC Packages" by Zemo Yang and Young Kwon in IEEE 1996 Electronic Components and Technology Conference, pages 165-171. According to this method, the surface temperatures of parts, substrates, and heat radiation equipment are first found by using a thermocouple, the junction temperature of a semiconductor package is estimated by using the voltage drop across the base and emitter of a transistor, and these temperatures are inserted in the various compact thermal resistance models shown in FIGS. 1A-1C. The thermal resistance of a semiconductor package itself has conventionally been found by methods using such compact thermal resistance models, and the thermal resistance thus determined are reflected in the design of cooling equipment.
Since heat generation in electrical components such as semiconductor packages causes problems in electronic components as described in the foregoing description, radiation fins are often mounted on the surface of case of semiconductor package to release the heat of the semiconductor package. However, radiation fins are not included in the above-described compact thermal resistance models shown in FIGS. 1A-1C. In other words, the thermal resistance of a semiconductor package with radiation fins mounted cannot be determined by the prior-art method of finding thermal resistance by using these compact thermal resistance models. The prior art includes cases in which the thermal resistance of a semiconductor package without radiation fins and the thermal resistance of radiation fins were each found separately, but thermal resistance in a case in which the two are unified has not been considered, and a calculation method has not been established. Mounting radiation fins on the case should change the thermal resistance of the path passing through the surface of case on which radiation fins are mounted, but this altered thermal resistance has not been found in the prior art. Although cooling devices may be designed based on the thermal resistance of a semiconductor package unit lacking radiation fins and the thermal resistance of the radiation fins unit, such an approach may not realize the optimum design.
In the above-described method of the prior art, moreover, the thermal resistance of a semiconductor package is found based on the measurements of various temperatures for one type of semiconductor package, and a change in any one of the factors that influence thermal resistance, such as the size of the semiconductor package, the size of the semiconductor chip, and the resin material that fills the inside of the semiconductor package, necessitates the repetition of the measurement of temperatures and associated calculations. The measurement results and calculation results for the thermal resistance computation relating to one type of semiconductor package therefore cannot be applied to another type of semiconductor package. Each and every measurement and calculation of the temperatures of the various points of the semiconductor package must be carried out whenever calculating for electronic components that are not exactly the same. The preparatory work necessary for designing electronic equipment is therefore extremely complicated, and this complexity both increases manufacturing costs and lengthens the time necessary to design and manufacture electronic equipment. The problem therefore exists that conventional methods cannot keep pace with the short life cycles of the recent products.
Although not described in detail, there is a method of finding the thermal resistance of a semiconductor package by a three-dimension heat-fluid simulation in which a three-dimensional problem of heat and fluid is solved by making use of a difference method or finite-element method based on the laws of conservation of mass (continuity equation), conservation of momentum (Navier-Stokes equation), and conservation of energy (conservation of energy equation). This method can be applied to determination the thermal resistance of a semiconductor package with radiation fins. In this method, however, as with the above-described prior art, the three-dimensional heat-fluid simulation must be repeated from the beginning if there is any change in any one of the factors that affect thermal resistance, such as the size of the semiconductor package or semiconductor chip or the material of the resin. If absolutely identical semiconductor packages are not used, therefore, the design of each electronic component necessitates extremely laborious procedures such as measurements of temperatures and calculations, meaning that a great deal of time is required for the design and manufacture of electronic component, and the resulting products are not sufficiently adaptable for the ever-shortening life cycle of product. In particular, methods in which such a three-dimensional heat-fluid simulation is carried out require special expertise in order to apply a difference method or finite-element method to partition meshes or set boundary conditions. These methods are therefore not easily used by anyone lacking expert knowledge, and in addition, are difficult because each and every electronic component must be individually designed.